What about having support for pEp for PGP encrypted emails like Thunderbird/Enigmail already has? It’s free and open source so it shouldn’t be problem to integrate it.
https://tools.ietf.org/html/draft-birk-pep
https://tools.ietf.org/html/draft-marques-pep-email
https://tools.ietf.org/html/draft-marques-pep-rating
https://tools.ietf.org/html/draft-birk-trustwords
https://tools.ietf.org/html/draft-marques-handshake
https://pep.foundation/pep-software/index.html